1. Field of the Invention
The present invention is concerned with the manufacture of insulation products based on mineral fibers.
2. Discussion of the Background
Insulating products based on mineral fibers are based on fibers obtained, for example, by centrifugal drawing. The fibers are joined together following a treatment which gives, among other things, the specific properties desired for the insulating products.
This treatment can be carried out in different ways. One can, for example, first assemble the fibers on a collector piece, such as a conveyor, compress them into a mattress, and then soak them in aqueous solutions containing thermoplastic resins or amino-plastics. Using this process water must be eliminated from the product by heat treatment. This leads to high energy costs.
Thus, preferably, one sprays the fibers the moment they are projected from the centrifuge with an aqueous solution based generally on phenoplastic resins, aminoplastics, or a mixture of the two In this process, the water is partially vaporized, and the last heat treatment serves essentially to reticulate the resins and to place the connected fibers in the form of a mattress.
An aqueous resin solution appropriate for use in a sizing composition must possess good stability as a function of time and must be highly tolerant of water. In this document, a sizing composition is understood as being an aqueous composition of a resin such as may be obtained from the condensation of starting materials from which a resin is obtainable as is well known in this art. Alternatively, a sizing composition is a composition to which has been added the usual sizing additives as well as, if needed, supplementary urea.
Solubility as used in this document is defined as follows. The solubility in water in the case of an aqueous resin solution is the volume of de-ionized water which can, at a given temperature, be added to a unit of volume of this solution before causing permanent trouble, e.g., the formation of a permanently heterogeneous system.
Aqueous solutions currently used, notably those based on aminoplastic resins, suffer from a water tolerance which can be insufficient in certain cases. Further, in the preparation of insulating shells, where the forming temperature is of the order of 80.degree. C., the low solubility of these aqueous resin solutions (as shown by the fairly short jellying time for the sizing solution in which the aqueous resin composition is diluted) poses the risk of insufficient adherence for fibers impregnated with these solution. The binder is drier, and the later heat treatment leads to inhomogeneous polymerization. Fibers impregnated in this manner have their mechanical properties changed due to difficulties of using sizing compositions on an industrial line. It is thus clear that sizing compositions play an essential part in the final properties of the insulation product because they directly affect the mechanical characteristics of the product.
Compositions based on resins of the melamine-urea-formaldehyde type have been used in sizing compositions for mineral fibers. These compositions, prepared according to usual methods by the condensation of formaldehyde, urea and melamine in the presence of a catalyst, are however often unstable over time and/or as discussed above, possess solubility characteristics which are insufficient to assure good dispersion of the resin on the fibers.
This is why normal resin-based gums of this type cannot be applied onto heated mineral fibers, by spraying. The short jellying time of these solutions leads to the unequal application of the binder on the fibers. Only an application by soaking layers of mineral fibers in the sizing composition is satisfactory with aminoplastic resins to obtain satisfactory fiber sizing. This limitation restricts the possibilities of using fibers joined by a sizing solution containing known aminoplastics. Notably such behavior excludes pipes for thermal insulation destined to carry fluids carried at high temperatures, of the order of 400.degree. C. In effect, for this type of application, compacting and molding of fibers is necessary to prepare tubular sheaths which correctly insulate the pipes. With napped fibers, it is not possible to carry out such compacting and molding operations.
There is thus a strongly felt need for an aqueous resin solution which can be used to prepare sizing compositions which do not possess the drawbacks found with currently existing aqueous resin solutions. Such an aqueous resin solution should possess good temporal stability. It should possess a high tolerance to water. And it should possess jelling time characteristics enabling it to satisfactorily impregnate mineral fibers at elevated temperatures.